The invention relates to a tripod bearing device with a stationary frame and a carrier moveable relative to it. These are connected together via three braces which are adjustable in length. Furthermore, the invention relates to a method of torsion compensation in the above device.
These types of devices and methods are used in machine tools for accommodating and moving a tool which generally can be fastened to the carrier and positioned by a relative movement with respect to a workpiece held on the frame. Further fields of application for tripod bearing devices are handling systems and robots where, for example, a manipulator, clamping device or gripper can be fitted to the carrier.
In contrast to conventional, three-axis tool guidance systems in which a dedicated, driven slide is provided for each linear axis with these slides being connected in series, tripod bearing devices consist of so-called parallel structures in which generally not just one, but a number of positioning mechanisms, for example braces which are adjustable in length, must be operated for a linear movement in an axial direction.
So-called hexapods, which feature a six-legged suspension of the carrier on the frame, belong to the field of parallel structures. A hexapod machining center and a device and method for controlling the same are known from DE 196 36 100 A1 and DE 196 36 102 A1 from the same applicant.
In the arrangement of a machine tool on the carrier, the hexapods mentioned facilitate five-axis machining on a workpiece fixed to the frame. Here, translations in all spatial directions as well as swivel movements are possible around axes running perpendicular to the spindle axis of the machine tool. With such hexapods the individual braces are exclusively loaded in tension and compression so that it is possible to use relatively long braces which exhibit a relatively low torsional stiffness which is favorable with hexapods with regard to as free as possible carrier movement in space from the viewpoint of brace collisions.
However, due to their complexity, hexapods are not only expensive with regard to their structure, but also with regard to their control and therefore they are of less benefit for applications in which a lower level of machining versatility is adequate.
With applications in which only three-axis machining is required, there is the possibility of reducing the number of braces and also therefore the complexity of the device compared to a hexapod. The complications that occur here from the linking of rotary degrees of freedom prevent the usual design principles applied to hexapods from being easily applied to structures with fewer braces. Through the linking of the rotary degrees of freedom, and in contrast to hexapods, tensile and compressive forces arise as well as torsion moments for which the braces usually used with hexapods and which have less torsional stiffness are unsuitable. A more torsionally stiff version would however negatively impair the movability due to the spatial restrictions, in particular on a compact carrier.
With regard to the torsion problem, a suggestion has already been made that with a tripod bearing device, i.e. a device with three braces, each brace should be formed as double rods so that moments that occur can be subdivided into tensile/compressive force components. However, these solutions use braces which cannot be varied in length and with which the positioning of the carrier is realized by offsetting the base point on the frame relative to the braces. This type of solution for braces adjustable in length is however impracticable.
Braces, non-varying in length with base-point guidance, also exhibit the disadvantage that errors in the bearing of the carrier are introduced due to the base-point guide tracks. These errors can only be compensated with difficulty and expense.
In contrast, the object of the invention is the creation of a tripod bearing device with braces adjustable in length which enables a high accuracy of positioning with low technical complexity.
With the tripod bearing device mentioned at the start, this object is solved in that each brace is coupled to the frame and to the carrier with a joint with two rotational degrees of freedom and a torsion compensation control device, with the torsion drives each allocated to the individual braces, interacts to twist the carrier-end joint of the relevant brace relative to the frame-end joint of this brace about its longitudinal axis for stabilizing/controlling torsion occurring in the braces.
With the solution according to the invention the torsions occurring due to the torsion drives of the individual braces in the parallel structure, which lead to a rotational deviation of the carrier from its set position, can be compensated in a targeted manner, and preferentially reduced to zero. In particular the displacement of a main spindle provided on the carrier, which occurs due to machining forces, can be compensated, giving a significantly higher stiffness of the carrier bearing system compared to conventional devices without having to carry out structural reinforcing on the braces. Consequently, a higher movability of the carrier in space can be realized even with a spatially compact carrier. Additionally, manufacturing accuracy and thermally influenced dimensional changes can be compensated by the controller in all six degrees of freedom. In contrast, with conventional three-axis serial structures this is only possible for the three translational degrees of freedom.
According to an advantageous embodiment of the invention, the torsion drive between the carrier-end joint and the frame-end joint is preferentially positioned at the carrier end of the brace. The latter is particularly of advantage when the torsion drive is light in weight compared to the brace. With small masses at the carrier end high acceleration levels can be achieved, giving a higher manufacturing speed.
Preferentially, the torsion drive is formed as a rotational drive device for small distances with a high driving speed. The rotational drive here represents a supplementary drive to the translational main drive of the braces which can move large distances with a relatively large dead-weight. This produces a high cut-off frequency for the complete arrangement.
According to an advantageous embodiment, the carrier is supported on the braces in space in true orientation, i.e. with a movement of the carrier in space, it always retains its alignment to the translational spatial axes.
According to another advantageous embodiment, a device for determining a torsion of the relevant brace about its longitudinal axis is provided on each brace for the generation of a signal in dependence of which the torsion drive of the relevant brace is operated. Through the direct acquisition of the actually occurring torsions they can be compensated on-line during the operation of the tripod bearing device, i.e. during a machining process. In principle the torsion drives can also be employed to a certain extent for the generation of torsions, provided that within the scope of a machining process slight swivel movements about the translation axes are desired.
Following another advantageous embodiment, an open-loop and/or closed-loop device for changing the lengths of the braces incorporates a torsion compensation control device which includes a closed-loop controller and the device for determining the torsions of the braces. Here, a system deviation is connected to the input end of the closed-loop controller, formed from parameters representing the torsion set values and parameters representing torsion actual values from the torsion determination device, and the output of the closed-loop controller is connected to the torsion drives. In this way disturbance quantities arising, such as moments resulting from a machining process, can be compensated in the desired way depending on the configuration of the closed-loop controller. The closed-loop control system can be realized analogue or digitally, with the former offering the advantage of a larger system dynamic response.
In contrast to hexapods, with tripods it is often possible to determine a closed analytical solution of the forwards transformation. This means that in the case of a digital closed-loop control system, the sampling rate and therefore the control quality can be improved.
The higher level open-loop and/or closed-loop control system just gives set values, such as for example offset quantities from the calibration or the thermal displacement of the system to the rotational drives, whereby these quantities exhibit more of a static character and only change very slowly.
Following another advantageous embodiment, the feedback of the parameters representing the torsion actual values to the input end of the closed-loop controller occurs autonomously without the motion control and/or closed-loop control of the brace length alteration being affected. For this, a cascade-type control concept, to which the torsion compensation closed-loop controller is arranged autonomously subordinate, is used for the change of length in the braces.
The object mentioned at the start is further solved by a method of torsion compensation in a device according to one of the claims 1 to 12 in which torsions caused by the load and arising in the braces are acquired, compared with set torsion values and the determined torsion difference is compensated by activating the torsion drives. This method enables the dynamic stiffness of a tripod bearing device to be increased in an economic way, thereby improving the production accuracy, without the movability of the device being impaired.